1. Field of the Invention
The present invention relates to a cathode-ray tube, and more particularly to an electron gun for use in a cathode-ray tube.
2. Description of the Prior Art
Recently available color picture tubes employ deflection yokes of the self-convergence type.
As shown in FIG. 1 off the accompanying drawings, such a self-convergence deflection yoke produces a horizontal deflecting magnetic field with pin-cushion distortion and a vertical deflecting magnetic field with barrel distortion for deflecting and automatically converging three R, G, B electron beams on a phosphor screen.
Since, however, the horizontal and vertical deflecting magnetic fields are distorted in the shapes of a pin-cushion and a barrel, respectively, the spot which is produced by the electron beams on the phosphor screen tends to be defocused or distorted at outer edges of the screen, as shown in FIG. 2 of the accompanying drawings. The electron beam spot is distorted because each of the electron beams, which has a certain finite spatial extent, is subjected to different forces in different locations on the phosphor screen.
The electron beam spot distortion, at an end of the X-axis of the phosphor screen, in the horizontal deflecting magnetic field, which is distorted in a pin-cushion pattern, will be described in greater detail with reference to FIG. 3 of the accompanying drawings. In FIG. 3, an electron beam e passes through the plane defined by FIG. 3 in a direction away from the viewer, and four 90.degree.-spaced points A, B, C, D are assumed to be on the peripheral edge of a cross-sectional plane through the electron beam e. Since the magnetic field is stronger at the point B than at the point A, the electron beam e undergoes lateral forces on its opposite sides. At the same time, forces directed toward the center of the electron beam e are applied to the points C and D.
Therefore, the electron beam spot on the phosphor screen is slightly underfocused, i.e., it would come to a focus beyond the phosphor screen, in the horizontal direction, and is strongly overfocused, i.e., it would come to a focus short of the phosphor screen, and hence diverges to produce a halo, in the vertical direction. FIGS. 4A and 4B of the accompanying drawings schematically show, using an optical lens system simulating the electron gun, how the electron beam is focused at the center and the X-axis end, respectively, of the phosphor screen. The optical lens system includes a main lens 31 and a deflection yoke 32. In FIGS. 4A and 4B, the electron beam is emitted from an object point a on a cathode, and is focused at a focus point f. The vertical lens effect of the optical lens system is shown on the upper side of the Z-axis, and the horizontal lens effect of the optical lens system is shown on the lower side of the Z-axis. The above horizontally underfocused and vertically overfocused condition of the electron beam spot is illustrated in FIG. 4B.
The relationship between the size of the electron beam spot and the focusing voltage applied to the deflection yoke is shown in FIGS. 5A and 5B of the accompanying drawings.
At the center of the phosphor screen, as shown in FIG. 5A, focusing voltages Vfv and Vfh applied to bring the electron beam spot into focus vertically and horizontally are equal to each other. The minimum sizes of the electron beam spots in the vertical and horizontal directions are the same as each other. Therefore, the electron beam spot is substantially circular in shape at the center of the phosphor screen.
At the X-axis end, however, the focusing voltage Vfv applied to focus the electron beam spot vertically is higher than the focusing voltage Vfh applied to focus the electron beam spot horizontally by .DELTA.Vfo (about 1.3 kv in FIG. 5B). Furthermore, the minimum sizes of the electron beam spots in the vertical and horizontal directions are different from each other; the horizontal minimum size of the electron beam spot is about 2.5 times greater than the vertical minimum size of the electron beam spot. The voltage difference .DELTA.Vfo is referred to as an astigmatic difference. The corrective voltage applied in a system which employs a dynamic quadruple structure and a dynamic focusing action (described hereinafter) is proportional to the astigmatic difference .DELTA.Vfo.
Since the electron beam spot comes to the focus f short of the phosphor screen in the vertical direction as described above, a halo is generated above and below the electron beam spot at the peripheral region of the phosphor screen, as shown in FIGS. 2 and 4B. As a result, the electron beam spot is distorted due to astigmatism at the peripheral region of the phosphor screen.
Cathode-ray tubes with non-self-convergence deflection yokes usually have a quadruple convergence yoke disposed behind the deflection yoke. The quadruple convergence yoke is supplied with a predetermined current in synchronism with the deflection of the electron beam by the deflection yoke. Usually, the electron beam spot in such cathode-ray tubes is also distorted at the peripheral region of the phosphor screen in the same fashion as with the self-convergence deflection yokes.
One solution to the above problem, employed particularly for low-cost cathode-ray tube models, is to make a portion of the electron gun rotationally asymmetrical to produce an astigmatic effect on the electron beam which is opposite to the astigmatism due to the deflection magnetic field for thereby improving the electron beam spot at the peripheral region of the phosphor screen. Inasmuch as the generated reversal astigmatic effect is fixed, the electron beam spot is necessarily brought out of focus at the center of the phosphor screen.
On the other hand, expensive cathode-ray tube models have an electromagnetic or electrostatic quadruple element near the main lens of the electron gun. The intensity of the converging effect of the quadruple element and the intensity of the focusing effect of the main lens are varied in synchronism with the deflecting action for producing a well-focused electron beam spot on the phosphor screen. Such a system is based on a combination of a dynamic quadruple structure and a dynamic focusing action. More specifically, the intensity of the converging effect of the dynamic quadruple element and the intensity of the focusing effect of the main lens are dynamically adjusted by a circuit arrangement to improve the focus of the electron beam spot at the peripheral region of the phosphor screen while maintaining the electron beam spot in focus at the screen center.
Actually, the above system is supplied with an AC voltage whose waveform is of a quasi-parabolic shape for improving the focus of the electron beam at the peripheral region of the phosphor screen. Since the astigmatic difference .DELTA.Vfo is large, as described above, it is customary to add an AC voltage of about 1 kv to the focusing voltage, which is normally in the range of from 5 to 10 kv. Because of the high voltage requirement, the required circuit arrangement suffers a relatively large burden.
Recently developed cathode-ray tubes for use in EDTV receivers, HDTV receivers, and computer display units employ higher deflection frequencies. As the corrective voltage is high, it is difficult to generate the voltage with a suitable waveform in view of the higher deflection frequencies without a complex circuit design and a high circuit cost.